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Critical Area 10% Rule Guidance Manual

Within the Critical Area there are three land use classifications:

Resource Conservation Areas (RCA)

Limited Development Areas (LDA), and

Intensely Developed Areas (IDA).

Most jurisdictions use these classifications as overlay zones. Intensely Developed Areas are generally areas
that were developed with residential, commercial, industrial, and institutional
land uses at the time of the original Critical Area mapping and where relatively
little natural habitat occurred. IDAs are also considered the preferred
locations for future growth through redevelopment and/or new development.

The Criteria set forth in conjunction with the Critical Area Act require that
any development or redevelopment within the IDA be accompanied by practices to
reduce water quality impacts associated with stormwater runoff. The Criteria
further specify that these practices must be capable of reducing stormwater
pollutant loads from a development site to a level at least 10% below the load
generated by the same site prior to development. This requirement is commonly
referred to as the “10% Rule.”

The responsibility of implementing the Criteria is delegated to each local
government. Therefore, each jurisdiction must ensure that the 10% Rule is met
for development projects located within the IDA. In order to provide a
consistent approach to compliance with the 10% Rule, the Critical Area
Commission provides guidance that includes a methodology for determining a
pollutant removal requirement and for quantifying the pollutants removed by a
variety of stormwater best management practices. This guidance, entitled
“Maryland Chesapeake and Atlantic Coastal Bays Critical Area 10% Rule Guidance”
is the result of revisions to prior publications printed in 1987 and in 1993.
The current guidance was reviewed and officially adopted by the Critical Area
Commission on December 3, 2003.

The current guidance reflects some significant changes in strategies for
addressing stormwater management and treatment in Maryland. The guidance
references and incorporates information included in the Maryland Department of
the Environment (MDE) 2000 Maryland Stormwater Design Manual, Vol. I & II. The
guidance addresses some of the inherent differences between the State’s
stormwater management program and the Critical Area 10% Rule, and provides
updated information for handling unique development situations.

It is important to note that this guidance information applies to development
and redevelopment of properties located within the Critical Area and designated
as an Intensely Developed Area (IDA). Some of the information and concepts
presented in this document may not be applicable to properties designated as
Limited Development Area (LDA) or Resource Conservation Area (RCA).